1. Field of the Art
This invention relates to a water-absorbing rubber composition comprising (A) a 1,3-diene rubber with a crystal or glass region content as measured at an ordinary temperature of 5-50% and (B) a highly water-absorbing resin dispersed in said rubber, said resin being in the form of a powder capable of passing through a 20-mesh Tyler screen and absorbing 20-500 times its own weight of distilled water. The composition can rapidly absorb water and show a high swelling pressure with excellent durability in the highly swollen state (ability to maintain a high swelling pressure for a long period of time), and is therefore especially useful as a water leak stopper or sealant to which a high swelling pressure is essential.
2. Description of the Prior Art
Compositions wherein a natural or synthetic rubber is mixed with a water-soluble polymer, which serves as a water-absorbing substance, such as polyvinyl alcohol, polyacrylic acid carboxymethylcellulose or hydroxyethylcellulose, are known. Such compositions can absorb water, whereupon they are swollen. Therefore, they are useful as water leak stoppers. However, since the water-absorbing substance used therein is water-soluble, the said compositions cannot be swollen to a high degree, hence can hardly produce a sufficient swelling pressure as required for the use as water leak stoppers, for instance. They cannot produce an early water-leak-stopping effect because of slow rates of water absorption. Attempts to use them as water leak stoppers in the practice consequently have encountered a number of difficulties.
Compositions for use as water leak stopper, wherein a water absorbing resin such as a polyurethane resin prepared from an isocyanate prepolymer is mixed with a natural or synthetic rubber are also know from the disclosure of Japanese Patent Applications laid open under Kokai Nos. 78-143,653, 79-7,461, 79-7,463 and 79-20,066, for instance. Also, Japanese Patent Application Kokai No. 78-30,654, for instance, discloses the use as a water leak stopper of a reaction product from an isocyanate prepolymer and a water-soluble polymer such as polyvinyl alcohol or polyacrylic acid. However, the compositions disclosed in the patent specifications cited above are not practical because they can swell in water at most three times, that is only to an insufficient extent for their use as water leak stoppers.
For eliminating the drawbacks as mentioned above, more recent Japanese Patent Applications Kokai Nos. 79-94,525 and 79-110,262 propose the use as water-absorbing substance of a highly water-absorbing resin capable of absorbing 10-200 times its own weight of water, which resin is incorporated into a rubber such as acrylic rubber, styrene-butadiene rubber (SBR), butadiene rubber, nitrile rubber, butyl rubber, or natural rubber, to give a water-absorbing rubber composition allegedly usable as a water leak stopper. However, such compositions cannot produce a high swelling pressure. Hence, the practical use thereof as a water leak stopper is still accompanied by a number of difficulties. Since the rubber used in the said composition is noncrystalline at an ordinary temperature, the rubber, when in the unvulcanized state, is subject to cold flow under a relatively weak external force. It is supposed that, even when the highly absorbing resin incorporated in said composition absorbs water and swells, the cold flow property of the rubber itself prevents production of a sufficient swelling pressure at the place to be sealed. It is also supposed that, even when the said composition is subjected to crosslinking, for example by vulcanization, to an extent sufficient to prevent cold flow, the network of crosslinked rubber molecules prevents swelling of the highly absorbent resin, so that a sufficient swelling pressure cannot be produced.
Japanese Patent Applications Kokai Nos. 80-131,035, 80-131,041 and 80-131,080 disclose the use as a packing material, for instance, of a composition prepared by mixing a natural or synthetic rubber or resin with a saponified starch-acrylonitrile copolymer, which is a highly water-absorbing resin. However, the said composition has a drawback in that its poor durability in the swollen state, supposedly due to the use of the said starch-derived highly absorbent resin, makes prolonged use of the composition impossible. When other resins than elastomers such as rubber are used, swelling by water absorption does not take place due to hardness or rigidity of the resins. Finally, Japanese Patent Application Kokai No. 80-99,986 discloses a composition prepared by mixing a rubber component with a hydrophilic resin comprising a (polystyrene)-(modified polybutadiene rendered hydrophilic by reaction with an anionic or cationic group-containing compound)-(polystyrene) type block copolymer, or a hydrophilic resin produced by a copolymerizing at least one hydrophilic monomer such as acrylic acid or an ester thereof with a polyol diacrylate. However, the use of the former hydrophilic resin as a highly water-absorbing resin results in production of a low swelling pressure as compared with the degree of swelling obtained when the latter hydrophilic resin is immersed in water, whereas the use of the latter hydrophilic resin results in an unsatisfactory degree of durability in the immersed and swollen state.